U.S. patent number 4,896,088 [Application Number 07/331,197] was granted by the patent office on 1990-01-23 for fault-tolerant switched reluctance machine.
This patent grant is currently assigned to General Electric Company. Invention is credited to Thomas M. Jahns.
United States Patent |
4,896,088 |
Jahns |
January 23, 1990 |
Fault-tolerant switched reluctance machine
Abstract
A switched reluctance motor drive or generator takes advantage
of the characteristic independence of concentrated phase windings
to optimize fault-tolerant operation. No "dead zones" in motor
torque production or generator voltage output are created by
faulted phases. One embodiment prevents unbalanced magnetic pull on
the rotor despite deactivation of a faulted phase by employing
multiple pairs of opposed stator pole windings in each phase.
Inventors: |
Jahns; Thomas M. (Schenectady,
NY) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
23292982 |
Appl.
No.: |
07/331,197 |
Filed: |
March 31, 1989 |
Current U.S.
Class: |
318/696; 318/685;
318/701 |
Current CPC
Class: |
H02P
9/02 (20130101); H02P 9/40 (20130101); H02P
25/08 (20130101) |
Current International
Class: |
H02P
25/02 (20060101); H02P 25/08 (20060101); H02P
9/02 (20060101); H02P 9/40 (20060101); H02P
9/00 (20060101); H02P 008/00 () |
Field of
Search: |
;318/696,685,701 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Langley, L. W. and Kidd, H. K., "Testing of a Redundant Motor
Designed for Space Shuttle Activation", Proc. of 1984 Nat.
Aerospace & Electronics Conf., May 1984, pp. 606-612..
|
Primary Examiner: Shoop, Jr.; William M.
Assistant Examiner: Bergmann; Saul M.
Attorney, Agent or Firm: Breedlove; Jill M. Davis, Jr.;
James C. Snyder; Marvin
Claims
What is claimed is:
1. A fault-tolerant switched reluctance machine drive,
comprising:
a multiphase switched reluctance machine including a rotor and a
stator, said rotor having a plurality of rotor poles, said stator
having a plurality of pairs of opposing stator poles, each phase of
said machine comprising at least one said pair of opposing stator
poles, each of said stator poles having a concentrated stator pole
winding wound thereon;
a plurality of independent power inverters, the number of said
inverters being equal to the number of said stator pole windings
corresponding to each machine phase, each of said inverters
comprising a power supply for impressing a DC voltage across said
stator pole windings, each of said inverters including at least one
current switching device connected in series with a separate one,
respectively, of said stator pole windings, the series combination
of each said stator pole winding and the corresponding
series-connected switching device comprising a separate respective
phase leg of the corresponding inverter; and
gate drive means coupled to each respective current switching
device for exciting the corresponding stator pole winding connected
in series therewith, the stator pole windings of said inverters
corresponding to the same machine phase having substantially the
same conduction interval, the conduction intervals for the
different machine phases being mutually time-shifted.
2. The switched reluctance machine drive of claim 1 wherein each of
said inverters is driven by a separate respective DC power
supply.
3. The switched reluctance machine drive of claim 1 wherein each
said phase leg further comprises at least one diode coupled to the
corresponding stator pole winding for circulating inductive winding
currents.
4. The switched reluctance machine drive of claim 1 wherein each
said phase leg includes a second current switching device connected
in series with the respective stator pole winding thereof.
5. The switched reluctance machine drive of claim 4 wherein each
said phase leg further comprises at least one diode coupled to the
corresponding stator pole winding for circulating inductive winding
currents.
6. The switched reluctance machine drive of claim 4 wherein each
said current switching device comprises a field effect
transistor.
7. The switched reluctance machine drive of claim 6 wherein each
said phase leg further comprises at least one diode coupled to the
corresponding stator pole winding for circulating inductive winding
currents.
8. The switched reluctance machine drive of claim 1 wherein the
poles of each said pair of opposing stator poles are diametrically
opposed.
9. The switched reluctance machine drive of claim 1 wherein each
machine phase further comprises a second pair of opposing stator
poles, the switched reluctance machine drive further comprising two
additional said power inverters, each said two additional inverters
including a phase leg corresponding to each respective machine
phase.
10. The switched reluctance machine drive of claim 9 wherein the
poles of each said pair of opposing stator poles are diametrically
opposed.
11. A fault-tolerant switched reluctance machine drive,
comprising:
a multiphase switched reluctance machine including a rotor and a
stator, said rotor having a plurality of rotor poles, said stator
having a plurality of pairs of opposing stator poles, each phase of
said machine comprising at least two said pairs of opposing stator
poles, each of said stator poles having a concentrated stator pole
winding wound thereon;
a plurality of independent power inverters, the number of said
inverters being equal to the number of said pairs of opposing
stator poles, each of said inverters comprising a power supply for
impressing a DC voltage across said stator pole windings, each of
said inverters including at least one current switching device
connected in series with a separate respective pair of stator pole
windings corresponding to each respective pair of said opposing
stator poles, the combination of each respective pair of stator
pole windings and corresponding series-connected switching device
comprising a separate respective phase leg of the corresponding
inverter; and
gate drive means coupled to each respective current switching
device for exciting the corresponding stator pole winding pair
coupled thereto, the stator pole winding pairs of said inverters
corresponding to the same machine phase having substantially the
same conduction interval, the conduction intervals for the
different machine phases being mutually time-shifted.
12. The switched reluctance machine drive of claim 11 wherein the
two stator pole windings of each said pair of opposing stator poles
are connected in series.
13. The switched reluctance machine drive of claim 11 wherein the
two stator pole windings of each said pair of opposing stator poles
are connected in parallel.
14. The switched reluctance machine drive of claim 11 wherein each
of said inverters is driven by a separate respective DC power
supply.
15. The switched reluctance machine drive of claim wherein each
said phase leg further comprises at least one diode coupled to the
respective pair of stator pole windings for circulating inductive
winding currents.
16. The switched reluctance machine drive of claim 11 wherein each
said phase leg includes a second current switching device connected
in series with the respective pair of stator pole windings.
17. The switched reluctance machine drive of claim 16 wherein each
said phase leg further comprises at least one diode coupled to the
respective pair of stator pole windings for circulating inductive
winding currents.
18. The switched reluctance machine drive of claim 16 wherein each
said current switching device comprises a field effect
transistor.
19. The switched reluctance machine drive of claim 18 wherein each
said phase leg further comprises at least one diode coupled to the
respective pair of stator pole windings for circulating inductive
winding currents.
20. The switched reluctance machine drive of claim 11 wherein the
poles of each said pair of opposing stator poles are diametrically
opposed.
Description
FIELD OF THE INVENTION
The present invention relates generally to fault-tolerance in motor
drives and generating systems. More particularly, this invention
relates to switched reluctance machines which have the capability
to continue operating with minimum performance degradation in spite
of machine or inverter faults.
BACKGROUND OF THE INVENTION
AC machines are not inherently fault-tolerant. The primary reason
is that the windings of AC machines are closely coupled
magnetically, so that a short circuit in one winding has serious
effects on adjacent phases. The problem is exacerbated in AC
machines having permanent magnets because rotating magnets excite
potentially dangerous high currents in any short circuit path.
Approaches to enhancing the reliability of AC motor drives and
generator systems generally involve the use of two or more AC
machines. For example, a common approach is to connect two or more
machines on a single shaft. Alternatively, gearing is used to
couple the machines together. However, there are weight, volume and
cost disadvantages associated with the use of additional machines,
thus making such approaches undesirable or even impractical for
many applications.
Another approach, which is described in U.S. Pat. No. 4,434,389,
issued to Langley et al., is to utilize redundant sets of
distributed windings, i.e., windings spread over a number of slots
around the air gap periphery. This approach, for machines energized
through an inverter, involves dividing a permanent magnet motor
into sections, each section comprising one set of
magnetically-coupled distributed windings. Each set of windings is
energized by a separate commutation circuit, so that the total
torque produced is the sum of the torques generated by each set of
distributed windings. For each motor section, a command unit
detects failures and removes the entire failed motor section from
service. Disadvantageously, the close magnetic coupling of the
distributed windings makes it necessary to disable the entire set
of section windings, even though the fault has developed in only
one of these windings. Thus, torque production is reduced by the
amount contributed by the entire motor section rather than by the
smaller portion delivered by a single winding.
In contrast to AC machines, a switched reluctance (SR) machine is
wound using concentrated windings, i.e., windings concentrated on
projecting motor poles. As a result, the phase windings of a SR
machine are essentially free of any magnetic coupling so that high
currents in one winding will not magnetically induce high currents
in adjacent phase windings. The present invention utilizes this
characteristic magnetic independence of switched reluctance machine
phases as the basis for a compact, fault-tolerant motor drive or
generator system. Such a fault-tolerant drive can be particularly
useful in aerospace applications for which highly reliable drives
are necessary.
Switched reluctance machines conventionally have multiple poles on
both the stator and the rotor; that is, they are doubly salient.
There is a concentrated winding on each of the stator poles, but no
windings or magnets on the rotor. Each pair of diametrically
opposite stator pole windings is connected in series or parallel to
form an independent machine phase winding of the multiphase SR
machine. Motoring torque is produced by switching current in each
machine phase winding in a predetermined sequence that is
synchronized with angular position of the rotor, so that a magnetic
force of attraction results between the rotor poles and stator
poles that are approaching each other. The current is switched off
in each phase before the rotor poles nearest the stator poles of
that phase rotate past the aligned position; otherwise, the
magnetic force of attraction would produce a negative or braking
torque. The torque developed is independent of the direction of
current flow, so that unidirectional current pulses synchronized
with rotor movement can be applied to the stator pole windings by
an inverter using unidirectional current switching elements, such
as transistors or thyristors. For use as a generator, the current
pulses in each machine phase winding are simply shifted so that
current flows when the rotor poles are moving past alignment
towards the unaligned position.
A SR motor drive or generator system operates by switching the
machine phase currents on and off in synchronism with rotor
position. That is, by properly positioning the firing pulses
relative to rotor angle, forward or reverse operation and motoring
or generating operation can be obtained. Usually, the desired phase
current commutation is achieved by feeding back a rotor position
signal to a controller from a shaft angle transducer, e.g. an
encoder or a resolver. However, in order to reduce size, weight and
cost in SR motor drives and generating systems, techniques for
indirect rotor position sensing have been developed, thus
eliminating the need for a shaft angle transducer. One such
technique is disclosed in commonly assigned U.S. Pat. No.
4,772,839, which issued on Sept. 20, 1988 to S. R. MacMinn and P.
B. Roemer.
Current regulators are typically employed for controlling phase
current amplitudes in a SR machine. There are several types of
current regulators. For example, individual low-resistance current
shunts may be coupled to each machine phase winding to detect the
current level in each phase. The output of each current shunt is
connected to a separate voltage comparator. Each comparator is also
connected to a separate potentiometer for setting the current
limit. Another type of current regulator, which eliminates the need
for discrete current sensors, is disclosed in U.S. Pat. No.
4,595,865, issued to T. M. Jahns on June 17, 1986 and assigned to
the instant assignee.
Commonly assigned copending U.S. patent application Ser. No.
304,159, filed on Jan. 31, 1989 by G. B. Kliman, S. R. MacMinn and
C. M. Stephens, discloses a system for detecting and isolating
faults in a SR motor drive, whereby faulted motor phases are
deactivated and motor operation is continued. More specifically,
this patent application, which is hereby incorporated by reference,
describes a SR machine fault management system which detects faults
through phase current differential sensing and phase flux
differential sensing. In addition, a method is provided for
starting the motor when stopped in a "torque dead zone" created by
a faulted phase. As used herein, the term "torque dead zone" is a
rotor angular position region in which positive motoring torque
cannot be produced by any of the intact non-faulted phases. By way
of contrast, in a SR generator system, a "voltage output dead zone"
is the counterpart to a torque dead zone in a SR motor drive. As
used herein the term "voltage output dead zone" is a rotor angular
position region in which no voltage output can be generated by any
of the intact non-faulted phases.
Although the hereinabove cited patent application advantageously
provides a system for isolating and detecting SR machine phase
faults, it is desirable to enhance the characteristic independence
of SR machine phase windings even further in order to optimize SR
machine fault-tolerant performance. In accordance therewith, it is
desirable to simplify the fault-tolerant SR machine drive and to
prevent the development of "torque dead zones" in motors and
"voltage output dead zones" in generators.
OBJECTS OF THE INVENTION
It is, therefore, an object of the present invention to provide a
new and improved switched reluctance motor drive or generator
system.
Another object of this invention is to provide a SR motor drive or
generator system which optimizes SR machine fault-tolerant
performance by taking advantage of the characteristic independence
of SR machine phase windings.
Another object of the present invention is to provide a
fault-tolerant SR motor drive or generator system which can
continue operating with minimal performance degradation despite the
existence of a fault in the machine or in its associated power
electronics.
Another object of the present invention is to provide a
fault-tolerant SR motor drive or generator system for which the
rotor does not experience an unbalanced magnetic force in spite of
the existence of a fault causing excitation to be removed from a
respective stator phase.
Still another object of the present invention is to provide a
fault-tolerant SR motor drive having no "torque dead zones" created
by faulted phases that prevent the intact phases from producing
torque in some rotor positions.
Yet another object of this invention is to provide a fault-tolerant
SR generator system having no "voltage output dead zones" created
by faulted phases that prevent the intact phases from generating
output power in some rotor positions.
SUMMARY OF THE INVENTION
In accordance with the present invention, a new and improved
switched reluctance motor drive or generator system is provided
with capability to continue operating with minimal performance
degradation in spite of the existence of machine or inverter
faults. To this end, the present invention utilizes the
characteristic independence of the concentrated phase windings of a
SR machine.
In one embodiment of a SR motor drive according to the present
invention, each stator pole winding is excited by a separate
respective inverter phase leg. For a SR motor having N phases and K
stator pole windings per phase (with K greater than or equal to 2),
this drive embodiment uses K independent inverters, with N phase
legs in each inverter. These inverters can be excited by the same
DC source or, preferably, by separate DC sources to achieve even a
higher level of fault tolerance. Loss of excitation to one stator
pole winding does not affect excitation of the remaining (K-1) pole
windings in the same phase, or excitation of any of the pole
windings in the other phases; therefore, average torque production
by the motor remains at approximately (NK-1)/NK of its normal,
pre-fault value. Moreover, no "torque dead zones" are created by
faulted phases in this new SR motor drive; that is, there are no
rotor positions for which the remaining intact phases cannot
produce torque. Hence, if the rotor is brought to a standstill
condition following a fault, no special controls are needed to
restart the machine.
In an alternative embodiment of a SR motor drive according to the
present invention, each motor phase comprises at least two pairs of
diametrically opposite stator poles. A stator pole winding is wound
on each pole, and the pole windings on diametrically opposite poles
are grouped together into pairs and connected either in series or
in parallel. For a SR motor having N phases and J stator pole pairs
per phase (for a total of 2NJ pole windings, with J greater than or
equal to 2,) this drive embodiment uses J independent inverters,
with N phase legs in each inverter. These inverters can be excited
by the same DC source or, preferably, by separate DC sources to
achieve even a higher level of fault tolerance. Loss of excitation
to one pair of diametrically opposite pole windings does not
substantially affect excitation of the remaining (J-1) pole winding
pairs in the same phase, or excitation of any of the pole winding
pairs in the other phases. Therefore, torque production continues
at approximately (NJ-1)/NJ of its pre-fault value, and no torque
dead zone is created. Advantageously, for this alternative SR motor
drive configuration, a fault in one inverter leg which results in
loss of excitation of one pair of stator pole windings will not
produce unbalanced magnetic pull on the rotor.
Further, according to the present invention, the machine
configurations described herein to realize fault-tolerant switched
reluctance motor (SRM) drives for delivering mechanical power to a
load also constitute fault-tolerant switched reluctance generator
(SRG) systems for converting mechanical power into electrical
power. Only the timing of the gating signals shifts with respect to
rotor position in order to convert a motor drive into a generating
system. Moreover, in a SRG system, voltage output dead zones, which
are the counterparts to torque dead zones in a SRM drive, are
eliminated by employing the fault-tolerant configurations of the
present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The features and advantages of the present invention will become
apparent from the following detailed description of the invention
when read with the accompanying drawings in which:
FIG. 1 is a schematic illustration of a conventional SRM drive;
FIG. 2 is a cross-sectional view of a SRM illustrating the
direction of current in an exemplary motor phase winding and
further illustrating the direction of magnetic flux resulting
therefrom;
FIG. 3 is a graphical illustration of the instantaneous torque
waveform for the SRM drive configuration of FIG. 1 following loss
of excitation of a faulted phase;
FIG. 4A is a cross-sectional view of a SRM constructed in
accordance with the present invention;
FIGS. 4B and 4C are schematic illustrations of the inverters
employed to drive the SRM of FIG. 4A;
FIG. 5 is a graphical representation of the instantaneous torque
waveform for the SRM drive configuration of FIG. 4;
FIG. 6A is a cross-sectional view of an alternative embodiment of a
SRM constructed in accordance with the present invention;
FIGS. 6B and 6C are schematic illustrations of a set of inverters
employed to drive the SRM of FIG. 6A; and
FIGS. 7A-7D are schematic illustrations of an alternative set of
inverters employed to drive the SRM of FIG. 6A.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a typical switched reluctance motor (SRM) drive
configuration. It is to be understood that the following
description of a switched reluctance motor drive is provided by way
of example only and that the principles of the invention apply
equally to switched reluctance generator systems. Therefore, as
used herein and in the claims, the term "machine" includes motors
and generators.
By way of example, SRM 10 is illustrated as a three-phase machine
with its associated power inverter 12. As shown, SRM 10 includes a
rotor 14 rotatable in either a forward or reverse direction within
a stationary stator 16. Rotor 14 has two pairs of diametrically
opposite rotor poles 18a-18b and 20a-20b. Stator 16 has three pairs
of diametrically opposite stator poles 22a-22b, 24a-24b and
26a-26b. Stator pole windings 28a-28b, 30a-30b and 32a-32b,
respectively, are wound on stator pole pairs 22a-22b, 24a-24b and
26a-26b, respectively. Conventionally, the stator pole windings on
each pair of opposing or companion stator pole pairs are connected
in series to form a motor phase winding so that the current I in
each phase produces a net magnetic flux linkage generating flux in
the directions indicated by arrows 52 and 53 in FIG. 2. For
example, as shown in FIG. 2, windings 28a and 28b are connected in
series so that the current flows in the direction indicated. As
illustrated in FIG. 1, the stator pole windings comprising each
companion pair 28a-28b, 30a-30b and 32a-32b, respectively, are
connected in series with each other and with an upper current
switching device 33, 34 and 35, respectively, and with a lower
current switching device 36, 37 and 38, respectively. The upper and
lower switching devices each comprise a field-effect transistor
(FET), but other suitable current switching devices may be used,
such as bipolar junction transistors (BJTs), gate turn-off
thyristors (GTOs) and insulated-gate bipolar transistors (IGBTs).
Each motor phase winding is further coupled to a DC power supply by
flyback or return diodes 45 and 42, 46 and 43 and 47 and 44,
respectively. At the end of each conduction interval of each phase,
stored magnetic energy in the respective motor phase winding is
returned to the DC source through the respective pair of these
diodes connected thereto. Each series combination of a motor phase
winding with two corresponding switching devices and two flyback
diodes comprises one phase leg of inverter 12. The inverter phase
legs are connected in parallel to each other and are driven by a DC
source, such as a battery or a rectified AC source, which impresses
a DC voltage +V.sub.S across the parallel inverter phase legs.
Capacitance 40 is provided for filtering transient voltages from
the DC source.
Typically, a shaft angle transducer 48 is coupled to rotor 14 for
providing rotor angle feedback signals to a motor control means 50.
However, as hereinabove discussed, techniques are available for
eliminating the shaft angle transducer. Phase current feedback
signals are supplied to control means 50 from a current regulator
(not shown), also hereinabove discussed, which receives phase
current feedback signals from current sensors (not shown). An
operator command, such as a torque command, is also generally
inputted to control means 50. In well known fashion, such as
described in U.S. Pat. No. 4,739,270, issued Apr. 19, 1988 to S. R.
MacMinn and P. M. Szczesny and assigned to the instant assignee,
the control means provides firing signals to inverter 12 for
energizing the motor phase windings in a predetermined
sequence.
In operation, if a fault occurs in a machine phase or an inverter
phase of a conventional SRM drive such that excitation is lost to
two opposing or companion stator pole windings, a "torque dead
zone" is created by the faulted phase. Although rotor inertia can
carry the rotor through this torque dead zone once it is rotating,
special inverter controls are needed to restart the SRM if it stops
in this dead zone created by the faulted phase. Once rotating, the
torque dead zone cannot be eliminated by overexciting the remaining
intact phases.
FIG. 3 is a graphical illustration of the instantaneous torque
waveform (T) for the SRM drive configuration of FIG. 1 following
loss of a faulted motor phase. The lost torque contribution due to
the faulted phase is indicated by dashed lines 56. As illustrated,
the average torque production T.sub.AVE is approximately two-thirds
of its initial pre-fault value T.sub.0.
A fault-tolerant three-phase SRM drive according to the present
invention is shown in FIG. 4A. In the following description, all
stator pole windings which share the same magnetic relationship
with the rotor, such as companion windings 32a and 32b, are
considered part of the same machine phase regardless of whether
they are directly interconnected. Unlike the conventional SRM drive
of FIG. 1, the stator pole windings wound on opposing or companion
stator pole pairs are not connected in series. Instead, each stator
pole winding is excited by a separate respective inverter phase
leg. In the preferred embodiment, two independent inverters 60 and
62 are utilized, each comprising three phase legs. Preferably, each
inverter 60 and 62 is driven by a separate DC source to achieve a
higher level of fault tolerance than if one power source were used.
Alternatively, however, both inverters can be driven by the same DC
source. As shown, each respective phase leg of each inverter
excites one stator pole winding respectively. Thus a first phase
leg of each of inverters 60 and 62 excites stator pole windings 28a
and 28b, respectively; a second phase leg of each of inverters 60
and 62 excites stator pole windings 30a and 30b, respectively; and
a third phase leg of each of inverters 60 and 62 excites stator
pole windings 32a and 32b, respectively. Thus each phase leg,
respectively, of each inverter corresponds to one of the three
motor phases, respectively, of SRM 10.
During normal, non-faulted operation, each stator pole winding
comprising a companion pair conducts simultaneously during a
predetermined conduction interval. That is, they are excited
coincidentally for torque production throughout a common time
interval. Moreover, the polarities of the companion stator pole
winding pairs are arranged so that the magnetic flux patterns are
identical to those of the conventional SRM, as illustrated in FIG.
2. In this way, under non-faulted conditions, the new SRM drive
operates in the same manner as the conventional SRM drive of FIG.
1.
However, unlike the conventional SRM drive, if a fault occurs in an
inverter phase or a machine phase of the SRM drive of FIG. 4, then
no dead zone in torque production is created. For example, even if
excitation is lost to stator pole winding 28a due to a fault,
uninterrupted excitation to the opposing or companion stator pole
winding 28b ensures that there nevertheless is some torque
production during the conduction interval of the corresponding
motor phase.
FIG. 5 is a graphical illustration of the instantaneous torque
waveform (T) for the SRM drive configuration of FIG. 4 following
loss of excitation to a stator pole winding of a faulted motor
phase. The torque contribution from the companion stator pole
winding of the faulted phase is shown by dashed lines 63. As
illustrated, because the companion stator pole winding of the
faulted phase still produces torque during the respective
conduction interval, there is no dead zone and the average torque
production T.sub.AVE is approximately 5/6 of the initial pre-fault
value T.sub.0, averaged over a complete rotation. Moreover, using
this configuration, the post-fault average torque may be increased
to the pre-fault value T.sub.0 if there is sufficient current
capacity to overexcite the remaining intact stator pole windings.
Advantageously, in the absence of a torque dead zone, no special
controls are required to restart the motor if the rotor stops
following a fault.
Under normal, non-faulted operating conditions, the excitation of
two opposing or companion stator pole windings with equal currents
ensures that the radial pull forces from the two corresponding
poles cancel, while their torque contributions add. However, when
excitation is removed from only one stator pole winding of a
companion pair, there is a net radial pull force on the rotor in
addition to the desired tangential force or torque. Therefore, it
may be necessary to reinforce the motor bearings to withstand the
resulting unbalanced magnetic pull on the rotor.
In an alternative embodiment of the present invention, generation
of the hereinabove described unbalanced magnetic force is
prevented. By way of example, FIG. 6 shows a three-phase SRM 70. As
illustrated, SRM 70 includes a rotor 72 within a stationary stator
74. Rotor 72 has four pairs of diametrically opposite rotor poles
74a-74b, 76a-76b, 78a-78b and 80a-80b. Stator 74 has six pairs of
diametrically opposite or companion stator poles 82a-82b, 84a-84b,
86a-86b, 88a-88b, 90a-90b and 92a-92b, respectively, fitted with
companion stator pole winding pairs 96a-96b, 98a-98b, 100a-100b,
102a-102b, 104a-104b and 106a-106b, respectively. In this example,
each motor phase comprises two pairs of diametrically opposing or
companion stator pole windings; i.e., two companion stator pole
winding pairs. For example, the two stator pole winding pairs
96a-96b and 102a-102b comprise one of the three motor phases of SRM
70. Preferably, two independent power inverters 105 and 107 are
employed to drive SRM 70. Each respective inverter phase leg
corresponds to a separate respective motor phase and comprises two
semiconductor switches and two flyback diodes which excite opposite
or companion stator pole windings connected in series with each
other. Alternatively, the two stator pole windings comprising each
companion pair, such as 96a-96b, can be connected in parallel. The
four stator pole windings corresponding to each respective motor
phase are excited for torque production during the same time
interval; i.e., they share an entire conduction interval in
common.
When a fault occurs in a motor phase of SRM 70 such that excitation
is removed from one pair of companion stator pole windings
corresponding to a respective motor phase, excitation is not
interrupted to the other companion stator pole winding pair.
Advantageously, therefore, in this embodiment of the SRM drive, the
fault does not create an unbalanced magnetic pull on the rotor or
its bearings since both diametrically opposed windings in the
faulted phase are unexcited. Moreover, if excitation is lost to
stator pole winding pair 96a-96b, for example, uninterrupted
excitation to the companion pair 102a-102b of that faulted phase
ensures that symmetrical excitation continues. Further, the average
torque production is reduced only by approximately 1/6 of its
pre-fault value for the same current, and no torque dead zone is
created by the fault.
Still another alternative embodiment of the inverter configuration
used to drive SRM 70 is shown in FIGS. 7A-7D. In this embodiment,
post-fault average torque is increased even further. As shown, four
independent three-phase inverters 110, 112, 114 and 116 are
employed. Each phase leg of each inverter corresponds to one
respective motor phase and excites one stator pole winding of a
companion pair corresponding thereto. Loss of one inverter phase
leg due to a fault removes excitation from only one stator pole
winding, resulting in loss of only approximately 1/12 of the
pre-fault average torque.
It is to be understood that the present invention is not limited to
three-phase SRM drives and SRG systems, but may be extended to SR
machines having any number of phases. Moreover, the present
invention is not limited to the numbers of stator poles and rotor
poles hereinabove described. For example, for a four-phase SR
machine having eight stator poles and six rotor poles, each of four
inverter phases can be used to excite two companion stator pole
windings corresponding to a respective machine phase.
Alternatively, each of the eight stator pole windings can be
excited by a separate inverter phase, the excitation of the four
stator pole winding pairs being synchronized during normal
operation.
While the preferred embodiments of the present invention have been
shown and described herein, it will be obvious that such
embodiments are provided by way of example only. Numerous
variations, changes and substitutions will occur to those of
ordinary skill in the art without departing from the invention
herein. Accordingly, it is intended that the invention be limited
only the spirit and scope of the appended claims.
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